1. Field of the Invention
This invention relates to main fuel pump for a gas turbine engine and particularly to a fuel pump which can be partially unloaded when the total flow of the pump is not required thereby preventing of excessive flow and heat from being generated.
The invention described herein was made in the performance of work under a government contract and the government has rights therein.
2. Description of Related Art
Modern gas turbine engines often employ positive displacement fuel pumps generally having one or two gear-type pumping elements and, as in the preferred embodiment of the present invention, may incorporate an integral centrifugal boost stage. Fuel pumps containing two gear stages may be connected in series or in parallel as in the present invention.
Conventional positive displacement fuel pumps are generally designed to supply more fuel than is needed for the engine's operation and particularly for partial power operation as during cruise and idle descent. The reduction in fuel pumping requirements is often referred to as the turn down ratio and can, for example, be on the order of 50 to 1 wherein the required pumping at idle descent is 2 percent of the amount required during takeoff. The fuel control determines the amount of fuel required for engine operation and bypasses the remainder back to the pump. This bypass flow is generally routed to the intake side of the high-pressure elements. A relief valve generally disposed in the discharge port of a conventional pump opens at a first predetermined pressure, for example 900 psi, and is capable of bypassing the total flow at a second higher predetermined pressure, for example 960 psi. This permits fuel at the pump discharge pressure to be recirculated as a protection against "dead-heading" the pump. The bypass fuel is routed to the inlet side of the two high-pressure elements.
The problem with such a fuel pump is that the redundant pumping results in increased heat load to the fuel system that must be cooled. This leads to the requirement of larger pumps than would otherwise be necessary. Fuel is often used in fuel to oil heat exchangers for cooling the engine's oil and the increased fuel temperature reduces its effectiveness for engine oil cooling thereby requiring larger or additional oil cooling heat exchangers which adds undesirable additional weight to the engine.